How to render an object as a transparent wireframe?
One way you could do this is by using freestyle:




Create a transparent material and assign it to your objects:


Enable freestyle in Render settings:


In Render layers, set the line visibility to QI range and set the end to 1:



Think of the QI range like a sort of depth index; faces directly visible to the camera are 0, faces behind those faces are 1, faces behind those faces are 2, etc.

By only rendering lines on faces between 0 and 1 (inclusive), you can get a result like in your example image.




Or you could use wireframe materials:



See this question for details.